A cathode-ray tube is evacuated to a very low internal pressure and accordingly is subject to the possibility of implosion due to the stresses produced by atmospheric pressure acting on all surfaces of the tube. This problem has been addressed in the art by providing the CRT with an implosion protection band. Such a band is used to apply a compressive force to the sidewall of a faceplate panel of the CRT to redistribute some of the forces. The redistribution of the forces decreases the probability of an implosion of the tube by minimizing tension in the corners of the panel. An implosion protection band also is beneficial because it improves the impact resistance of the tube. Glass in compression is stronger than glass which is in tension and the band causes compression in panel areas which otherwise would be in tension. Additionally, in the event of an implosion, the redistributed stresses cause the imploding glass to be directed toward the back of the cabinet in which the tube is mounted, thereby substantially reducing the probability of someone in the vicinity of the imploding tube being injured.
Steel is the preferred material for many types of implosion protection bands because of its low cost and high strength.
U.S. Pat. No. 4,121,257, issued to V. R. Krishnamurthy on Oct. 17, 1978, describes the use of zinc, zinc-and-epoxy, and plastic coatings for steel tension or "T-band" systems in which coated steel strapping is applied to the tube, then tensioned and crimped, to provide implosion protection.
Epoxy and plastic coatings overlying a steel base cannot be used with shrinkfit bands. A shrinkfit-type band may be manufactured from a single, continuous steel piece, from a steel strip joined at the two ends, or from a plurality of steel strips joined together at the ends. The band, in each instance, is formed into a loop, the periphery of which is smaller than the periphery of the faceplate panel. The loop is heated to approximately 300.degree. to 500.degree. C. and the coefficient of expansion of the steel causes the loop to expand to dimensions which permit the loop to be slipped around the sides of the faceplate panel. As the band cools it shrinks and tightly surrounds the panel, thereby applying an inwardly directed compressive force to the faceplate panel to offset at least some of the outwardly directed tension forces which are produced on the faceplate by atmospheric pressure, when the tube is evacuated. The elevated temperatures applied to the band to expand it would damage an epoxy or plastic coating on the band and possibly foul the shrinkfit banding apparatus. Accordingly, such coatings cannot be used to provide corrosion resistance to the steel band.
Applicants have determined that a zinc coating applied to the steel band also is unacceptable for shrinkfit bands, because the elevated temperature, applied to the band to expand it, causes the zinc to react with the iron of the steel band to form intermetallic compounds with poor corrosion resistance to the humidity conditions often experienced during the transport and storage of the tubes. Intermetallic compounds, as the term is used herein, are alloys of two metals in which a progressive change in composition is accompanied by a progression of phases, differing in crystal structure, through the material. In order to enhance the corrosion protection of the shrinkfit steel band, it is necessary to inhibit the formation of such zinc-iron intermetallic compounds.